TORONTO (AP) — The United States banned all beef imports from Canada after a lone case of mad cow disease was discovered in the heart of Canada’s cattle country on Tuesday.

The discovery raised concern because Canada and the United States had put in place feeding practices authorities thought would prevent the infection from spreading in North America. Still, officials stressed it was an isolated case.

“Information suggests that risk to human health and the possibility of transmission to animals in the United States is very low,” U.S. Agriculture Secretary Ann Veneman said in a statement.

The infected cow, an 8-year-old from a farm in northern Alberta, was slaughtered on Jan. 31, Canadian Agriculture Minister Lyle Vanclief told a news conference in Edmonton. He said it was killed because it was believed to have pneumonia, and testing in England confirmed Tuesday it had bovine spongiform encephalopathy, or BSE.

It was the first case of BSE in North America in a decade. He stressed the diseased animal never reached human or animal consumption, the known method of spreading the infection. It cannot be transmitted between live animals.

“The herd has been quarantined,” Vanclief said. “A trace on the animal is being done. The animal did not go into the food chain.”

The only previous known North American case of BSE, also called mad cow disease, was in 1993 involving an animal born in Britain, he said. That herd was destroyed and there was no further spread of the disease, he said.

Beef imports to the United States were not banned at the time because officials said it was just one cow and the source of the disease was Britain.

It was not immediately clear where the cow in the new case was born.

Reaction to the announcement was immediate. Canada voluntarily stopped issuing certificates that declare its cattle free of BSE, officials said, and U.S. authorities banned imports of Canadian cattle, beef, beef-based products and animal feed.

Shares of big U.S. hamburger chains fell sharply following the news.

Shares in McDonald’s, the world’s largest restaurant company, fell $1.02, or 5.6 percent, to $17.14 in late afternoon trading on the New York Stock Exchange. Also sinking were fast-food rival Wendy’s International, Jack in the Box and Tyson, the world’s largest meat company. Outback Steakhouse Inc. also fell.

U.S. companies quickly tried to put any potential consumer fears about their meat to rest.

“McDonald’s worldwide has the highest beef safety standards and will continue to strictly enforce them,” the Oak Brook, Ill., company said.

“McDonald’s Canada only purchases beef from facilities federally inspected and approved by the Canadian Food Inspection Agency.”

The company said it does not import beef from Canada. Outback Steakhouse said its restaurants serve only USDA top choice or prime U.S. Midwestern grain-fed beef.

On Wednesday, authorities in Australia said they had tracked down all Canadian cattle imported into Australia and banned further imports. Australia only imports breeding cattle from Canada and does not buy any meat products.

No case of mad cow disease has ever been found in U.S. cattle. The U.S. government routinely bans the import of meat and livestock from countries where mad cow disease is found.

In Hereford, Texas, John Josserand of AzTx Cattle Co. said the fact that Canadian safeguards detected the BSE case was encouraging. He said U.S. and Australian cattle producers could benefit in the short run from a ban on Canadian exports, “although that’s not the kind of competitive advantage I want.”

A note to investors from John Ivankoe, restaurant analyst for J.P. Morgan, was more cautious.

“The discovery of one cow is concerning, even though it did not enter the food chain, as the discovery of other cases is possible,” Ivankoe said. If only the Canadian industry is affected, he added, then it is a “relatively minor issue.”

However, an industry trade goup called for tighter U.S. checks on Canadian products, saying that the integration of North American economies puts all consumers at risk.

“In this day of industrialized beef production and liberalized trade, it is disingenuous to say this is about one isolated cow,” said Mark Ritchie, president of the Minneapolis-based Institute for Agriculture and Trade Policy.

Mad cow disease first erupted in Britain in 1986, and is thought to have spread through cow feed made with protein and bone meal from mammals.

The human form of mad cow disease is Creutzfeldt-Jakob disease, which causes paralysis and death. Scientists believe humans develop new variants of Creutzfeldt-Jakob when they eat meat from infected animals.

Both Canada and the United States outlawed the feeding of meat and bone meal to cattle, sheep and goats in 1997, a rule believed to be the main defense against the disease. The incubation period for BSE can be eight years, so the cow in Canada could have been infected by feed predating the ban.

Authorities will trace the origin of the cow as part of an investigation into any possible spread of the disease, Vanclief said. They have also quarantined the farm and will slaughter the herd of the infected cow, along with any other herds that come into question.

Alberta controls about 40 percent of Canada’s cattle industry, and the oil-rich province is one of the country’s wealthiest. It draws millions of visitors each year to the Canadian Rockies.

Canada is the top foreign supplier of live cattle to the United States, exporting 1.7 million head last year, or 75 percent of the total U.S. imports. About seven percent of beef consumed by Amercians is from Canada, said Michelle Peterson, a spokeswoman for the National Cattleman’s Beef Association, which represents U.S. beef producers.

The FDA and U.S. Agriculture Department are working with Canadian officials to get more information about the sick cow, including records concerning its past ownership and what animal feed it was given.

It is the second time this year a disease outbreak is threatening Canada’s economic well-being, after severe acute respiratory syndrome from Asia killed 24 people in the Toronto area in the past two months.

The World Health Organization removed Toronto from its list of SARS hotspots earlier this month.